Photolithography is a major process in the manufacturing of integrated circuits (ICs). A major task of a photolithography process is to transfer patterns on a mask to a photoresist layer on a wafer. The projection light propagates through the patterns on the mask, and arrives at the wafer. The patterns on the mask are equivalent to obstacles in the propagation path of the projection light, thus the patterns on mask may be obtained on the wafer. According to the principle of light diffraction and the principle of light interference, a light diffraction may happen when the projection light propagate through the mask; and a light interference may happen between the projection light at different position of the mask. Thus, the light intensity distribution of the actual projection light projecting on the wafer is a result of a superposition of diffracted light; and the patterns on the wafer may be unable to be exactly identical to the patterns on the mask.
According to the principle of light diffraction and the principle of light interference, when a size of the obstacles is far greater than a wavelength of the projection light, an error of the patterns caused by the light diffraction may be omitted. That is, when a size of the patterns (critical dimension of ICs) on the mask is far greater than a wavelength of the projection light, the patterns on the mask may be identical to the patterns on the wafer. However, for an ultra-submicron fabrication process, the critical dimension of the ICs may be bellow 0.13 μm, or even bellow 0.09 μm.
Thus, the critical dimension of the ICs may be near to, or smaller than the wavelength of the projection light, thus the light diffraction may become more prominent, and a size deviation between the patterns on the mask and the patterns on the wafer may no longer be omitted. With the continuously shrinking of the critical dimension of the ICs, deformations and deviations of the patterns formed by a photolithography process have become more and more severe, and have become a major issue affecting performance and yield of the ICs.
At positions where adjacent patterns are proximity to each other, the light diffraction and interference may become more obvious, and the pattern deviations may be relatively large. For example, the pattern deviations at the ends of lines or corners of patterns may be relatively obvious. Another example is an array of regular patterns (such as through holes, etc). When the projection light propagates through a pattern array on the mask, a light intensity along a lateral direction and a light intensity along a diagonal direction may not be symmetric, thus the corresponding pattern array formed on the photoresist layer may be deformed.
In the manufacturing of ICs, certain patterns may be key components affecting electrical properties and functions of circuits. Thus, if these patterns are deformed, the performance of the whole IC chip may be affected, and even cause the circuits to fail. The deviation between the patterns on the mask and the obtained patterns after the photolithography process caused by the light diffraction and/or light interference may be referred as an optical proximity effect (OPE). In a photolithography process, the OPE may be unavoidable. Thus, certain methods and techniques may have to be used to reduce the deformation and the deviation between the patterns on the mask layer and the corresponding patterns on the wafer as much as possible, so that the performance and yield of the IC chips may be guaranteed.
A yield-driven mask correction method may be implanted between the physical design of a mask and the fabrication of mask to reduce the OPE. For the yield-driven mask correction method, the deformation and deviation of the patterns during a photolithography process may be compensated by alternating shapes and/or densities of the patterns on the mask, and/or phases of the projection light propagating through the mask. Expected patterns may be obtained on the wafer using the yield-driven mask correction method. The yield-driven mask correction method may include a reticle enhancement technology. The reticle enhancement technology may include an optical proximity correction (OPC) and a phase shift mask. Among of these technologies, the OPC may be an effective reticle enhancement technology.
A correction method using assisting patterns, as a relatively mature OPC method, has been widely used. The correction method using assisting patterns include laying out main patterns (mask patterns) on the mask; followed by adding assisting patterns around the main patterns. When performing an exposure process, the main patterns are resolved from a photoresist layer on a wafer, while the assisting patterns are not resolved from the photoresist layer. When the assisting patterns are added, loose layout patterns and dense layout patterns may have a same layout environment, and it causes an exposure light propagating through the patterns on the mask to have same density distributions in different directions. Thus, it not only increases the process window of a photolithography process, but also increases a contrast of the patterns.
In row-column distributed patterns, assisting patterns are also often added to increase the contrast. FIG. 1 illustrates a corresponding layout.
As shown in FIG. 1, row-column distributed main patterns 101 are formed on a mask; and an assisting pattern 102 is added at a cross point of two diagonals of four main patterns 101. By adding the assisting pattern 102, after an exposure light propagates through the patterns on the mask, an asymmetry between a lateral light intensity Ix and a diagonal light intensity Ic is reduced, thus a deviation between the patterns in a photoresist layer and the expected patterns is reduced. That is, if there is no the assisting pattern 102, when the exposure light irradiates the main patterns 101, because the distance between two lateral main patterns 101 is not equal to the distance between two main patterns 101 along the diagonal direction, or the distance between two lateral main patterns 101 is significantly different from the distance between two main patterns 101 along the diagonal direction, the light diffraction and the light interference of the exposure light along the lateral direction and the diagonal direction are different. Thus, the intensities of the exposure light propagating through the main patterns along the lateral direction and along the diagonal direction are asymmetric, or significantly different; and the obtained patterns in the photoresist may be significantly different from the expected patterns.
With the continuously shrinking of the critical dimension of patterns, spaces between the patterns has become smaller and smaller. It may bring new challenges to the layout of the assisting patterns, and the process window of a photolithography process. For example, it may be difficult to add assisting patterns between adjacent patterns. The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems.